The present invention relates to a polarization split technique in a projection-type image display apparatus which forms an optical image according to an image signal by irradiating light valves such as liquid crystal panels with light from the light source side and enlarges/projects the optical image.
Business-use liquid crystal projectors have become widespread. In addition, as a substitute for the conventional projector which displays an image on a cathode-ray tube for projection to a screen, liquid crystal panel-used projection TVs have been developed. In particular, home-use projection TVs require higher fidelity color reproduction, higher contrast performance and faster motion image display performance than business-use liquid crystal projectors.
In the case of a reflective liquid crystal panel, it is possible to substantially halve its liquid crystal layer in thickness as compared with transmissive liquid crystal panel since the liquid crystal layer is passed back and forth, that is, passed twice in total due to reflection. Reducing the thickness of a liquid crystal layer to a half results in quadrupling the response speed. This is advantageous when motion images are displayed.
Generally, in a liquid crystal projector system employing such a reflective liquid crystal panel, polarization split means is provided in front of the reflective liquid crystal panel. Serving as both a polarizer and an analyzer, the polarization split means transmits waves polarized in a specific direction and reflects waves polarized perpendicular to the direction. Techniques of this kind are described in, for example, Japanese Patent Laid-open No. 2001-142028 and Japanese Patent Laid-open No. 2003-131212.
In these laid-open patents, three polarization split means are respectively combined with three reflective liquid crystal panels; one for red light, one for green light and one for blue light. The red, green and blue light rays are composed by a cross dichroic prism.
Examples of the polarization split means includes a PBS prism where a dielectric multilayered film serves as a polarizing beam splitter (hereinafter denoted as a PBS) is formed in an interface between two rectangular prisms (Japanese Patent Laid-open No. 2001-142028) and a wire grid type polarization split device which has a diffractive grating structure constructed by forming wire (metal) grid lines with a predetermined pitch (patterning period) on a glass substrate (Japanese Patent Laid-open No. 2003-131212).
The PBS prism described in Japanese Patent Laid-open No. 2001-142028 shows superior polarization split ability with large extinction ratio for a perpendicular incident light beam. However, if oblique light, not parallel to the plane (principal plane of incidence) formed by the optical axis and the line normal to the surface of the PBS film, is incident on the PBS prism, leakage light occurs lowering the extinction ratio. Although a quarter-wave plate is placed in front of each reflective liquid crystal panel to solve this problem, its effect is not necessarily possible to sufficiently raise the contrast.
In the case of the wire grid type polarization split device described in Japanese Patent Laid-open No. 2003-131212, although the peak value of the extinction ratio is low at an incident angle of 45 degrees, the extinction ratio does not show large deteriorations for oblique light beams as indicated by the polarization split characteristics of FIG. 4 in the laid-open patent. Total contrast performance for the flux of light is therefore good. In the wire grid type polarization split device, however, the following point must be taken into consideration.
As illustrated in FIG. 12, there are two ways of arranging a wire grid type polarization split device in an optical path along which a flux of light is reflected by a reflective liquid crystal panel and then enters a projection lens. In the arrangement method of FIG. 12(1), a flux of S-polarized incident light from an illuminating optical system is reflected by a wire grid type polarization split device 17 and then enters a reflective liquid crystal panel 214. The flux of light is changed to P-polarized light by the reflective liquid crystal panel 214. The outgoing light (reflected light) passes through the wire grid type polarization device 17 and goes to a projection lens (not shown in the figure) (For convenience, this arrangement is referred to hereinafter as “transmissive arrangement” since the reflected light from the reflective liquid crystal panel passes through the wire grid type polarization split device and goes to the projection lens.). In the arrangement method of FIG. 12(2), a flux of P-polarized incident light from an illuminating optical system passes through a wire grid type polarization split device 17 and enters a reflective liquid crystal panel 217. The flux of light is changed to S-polarized light by the reflective liquid crystal panel 217. The outgoing light (reflected light) is reflected by the wire grid type polarization device 17 and goes to a projection lens (not shown in the figure) (For convenience, this arrangement is referred to hereinafter as “reflective arrangement” since the reflected light from the reflective liquid crystal panel is reflected by the wire grid type polarization split device before going to the projection lens.).
In the reflective arrangement, a wire grid type polarization split device is placed as shown in FIG. 12(2). In this case, the projection performance may deteriorate if the wire grid type polarization split device gets out of position or the wire grid type polarization split device thermally expands/transforms. In the transmissive arrangement, a wire grid type polarization split device is placed as shown in FIG. 12(1). In this case, the projection performance may also deteriorate due to the astigmatism caused during transmission through the plate-shaped wire grid type polarization split device.
In Japanese Patent Laid-open No. 2003-131212, a wire grid type polarization split device is set in the transmissive arrangement as shown in FIG. 1 therein. In addition, to reduce the astigmatism peculiar to the transmissive arrangement, polarization split means is constructed by forming the wire grid type polarization split device in an oblique interface between two rectangular prisms as shown in FIG. 2 therein. In this case, the astigmatism is reduced since the glass substrate of the wire grid type polarization split device has almost the same refractive index as the rectangular prisms.
In this kind of polarization split prism (hereinafter denoted as a “diffractive prism”) constructed as polarization split means by forming a wire grid type polarization split device in an oblique interface between two rectangular prisms, it is possible to shorten the optical path length. This makes it possible to shorten the back focus of the projection lens and therefore miniaturize the projection lens. Further, since widening of the light beam can be reduced, it is possible to miniaturize the diffractive prism.
Almost the same effect can also be obtained by placing a wire grid type polarization split device in a rectangular translucent container filled with a liquid medium whose refractive index is substantially the same as the glass substrate of the wire grid type polarization split device.
The wire grid of a wire grid type polarization split device is heated since it absorbs 5 to 10% of the incident light. The temperature rise may cause birefringence in the translucent glass substrate due to thermal stress, which may lower the contrast. In addition, the temperature rise may thermally expand/transform the glass substrate which may cause deterioration in the projection performance as well. If the wire grid type polarization split device is set in a liquid medium, it is possible to suppress the temperature rise while providing the same effect as the above-mentioned diffractive prism.
Wavelengths of light in a wire grid type polarization split device placed in a medium (glass, ethylene glycol or the like) whose refractive index is larger than that of the air as in a diffractive prism are shorter than those in the same polarization split device used in the air. To secure appropriate polarization split performance for use in such a medium, it is therefore necessary to further shorten the lattice pitch of the wire grid.
A method of manufacturing a wire grid type polarization split device is described in paragraph 0039 of Japanese Patent Laid-open No. 2003-131212. In this method, an underlayer aluminum film is formed on a glass substrate and a resist pattern is formed thereon by electron beam lithography. Then, aluminum is vapor-deposited to a predetermined depth and unnecessary aluminum is selectively removed by the lift off method to form a metal grid. Thus, its dimensional accuracy depends on the performance of the electron beam lithography system. As for the minimum line width that can be drawn by existing electron beam lithography systems, typical systems allow line widths down to 100×10−9 m while high resolution systems allow line widths down to 30×10−9 m.
A typical commercially available example of a wire grid type polarization split device is from MOXTEK, Inc. In this product, a wire grid is formed with a line width of 6.5×10−9 m and a lattice pitch of 150×10−9 to 200×10−9 m. The glass substrate of the wire grid is 0.7×10−3 to 1.6×10−3 m in thickness. To obtain the same polarization split performance in a medium having a refractive index of 1.5 as in the air, it is necessary to reduce the line width to 43×10−9 m and the lattice pitch to 100×10−9 to 130×10−9 m. Taking the required dimensional accuracy into consideration, these values are difficult to realize even with a high resolution electron beam lithography system.
Reducing the lattice pitch of the wire grid to such a level faces manufacturing difficulties. Currently, commercially available wire grid type polarization split devices are assumed to use in the air.
FIGS. 13 and 14 show how the polarization split performance of a wire grid type polarization split device designed for use in the air changes if the device is placed in a mixture of ethylene glycol and glycerin (hereinafter denoted as “GE55) having a refractive index of 1.45. FIG. 13 concerns the transmission of P-polarized light while FIG. 14 concerns the transmission of S-polarized light. As apparent from FIGS. 13 and 14, if the wire grid type polarization split device designed for use in the air is set in the medium GE55, the transmission of P-polarized light to be transmitted decreases whereas the transmission of S-polarized light to be reflected increases. In the aforementioned transmissive arrangement, if the transmission of P-polarized light decreases, the brightness deteriorates due to a decrease in the amount of light going to the projection lens from the reflective liquid crystal panel. In addition, the contrast deteriorates since the transmission of the S-polarized light to be removed increases to lower the polarization degree.
In the case of a polarization split device which splits light by polarization by means of a diffracting grid structure, such as a wire grid type polarization split device, its polarization split performance deteriorates as mentioned above if the device is used in a medium whose refractive index is larger than the air. The present invention is directed to this problem of the related art technique.